1. Field of the Invention
The present invention relates to a magneto-optical recording medium such as a magneto-optical disk. It relates particularly to a magnet-optical recording medium for recording data thereon and reproducing data therefrom by application of laser light and magnetic field utilizing a magneto-optical effect.
2. Description of Related Art
In some prior-art magneto-optical recording media, data is recorded on a magnetic film by applying a laser beam from a substrate side, and the data recorded on the magnetic film is reproduced by applying a laser beam from the substrate side. For example, as shown in FIG. 4, a medium is comprised of a dielectric film 52 (of SiN), a magnetic film 53 (of TbFeCo), a dielectric film 54 (of SiN) and a heat radiation film 55 (of AlCr) formed in this order on a substrate 51 (of polycarbonate). A laser beam 61 is converged through an object lens 62 above the substrate 51 and applied onto the medium.
In this case, since the heat radiation film 55 is composed of a monolayer film and does not intervene between the substrate 51 and the magnetic layer 53, the physical structure of the heat radiation film 55 does not affect the reproduction of data from the magnetic film 53 directly.
On the other hand, there are prior-art magneto-optical recording medium which are irradiated with a laser beam from a side opposite to the substrate. These media are called first surface recording medium. FIG. 5 shows the construction of a prior-art recording medium of this type.
The first surface recording medium is composed, for example, of a heat-radiation film 55 (of AlCr), a dielectric film 54 (of SiN), a magnetic film 53 (of TbFeCo) and a dielectric film 52 (of SiN) formed on a substrate 51 (of polycarbonate) in this order. The heat radiation film 55 of this medium is also formed of a monolayer film, for which not only AlCr but also AlTi and Ag-containing alloys such as AgCuPd and the like have been used (see Japanese Unexamined Patent Publication Nos. 2000-109943 and 2000-285517).
In this first surface recording medium, since the heat radiation film 55 is first formed on the substrate 51 and thereafter the dielectric film 54 and the magnetic film 53 are formed, the magnetic characteristics of the magnetic film 53 are greatly affected by depressions and projections on the surface of the heat radiation film 55 underlying the magnetic film 53. Generally, since the heat radiation film 55 contains metal particles, there are formed significantly large depressions and projections on the surface of the heat radiation film 55, which are undesirable for the magnetic characteristics.
Therefore, the magnetic film 53 formed in the heat radiation film 55 also has depressions and projections, which results in an irregular magnetization direction. Preferably, the magnetization direction is as uniform as possible in a direction vertical to the surface of the film from the viewpoint of coercive force. A medium not having a uniform magnetization direction exhibits only a small coercive force and does not provide desired magnetic characteristics. Especially, the magnetic film cannot be improved in resolution. By simply thickening the heat radiation film for improving heat radiation, desired heat radiation property can be obtained, but the high resolution of the magnetic film cannot be obtained because of existence of large depressions and projections of the heat radiation film.
In order to achieve higher recording and reproducing density as currently required of recording media, the magnetic film needs to have higher resolution. For higher resolution, an S/N ratio (or a CNR) at recording and reproduction is required to increase. The S/N ratio is related to the heat radiation property of the heat radiation film, and in order to improve the S/N ratio, it is necessary to raise the heat radiation, i.e., to raise thermal conductivity.
However, in the prior-art first surface recording medium as shown in FIG. 5, since the heat radiation film is composed of a monolayer film, it is difficult to maintain or improve the heat radiation property and improve the resolution at recording and reproduction at the same time.